Several problems, all dealing with the relationship between growth and differentiation, are being investigated using the model unicellular microorganism, Tetrahymena, which can be synchronized by a physical selection procedure, and which can also be induced to withdraw from the growth cycle and enter the sexual phase of its life cycle. Conjugation, which occurs between cells only in the G1 phase of the cell cycle, can be induced by starvation. We are studying the mechanisms of induction, the surface changes required for cell pairing, the nuclear events during conjugation, and the relationship between the division cycle and the conjugation reaction. Cells can be synchronized by a physical selection procedure and can be directed to either replicate DNA and divide synchronously, or to conjugate without division. Within the division cycle we are investigating the link between gene replication and transcription for focusing on the replication of ribosomal cistrons and the rate of synthesis of ribosomal precursor RNA through the cell cycle. We intend to explore the phenomenon of alterations of the relative time of replication of rDNA caused by starvation since starvation is also the inductive signal for conjugation.